Multilayer pipe and die for manufacturing it

ABSTRACT

A multilayer pipe is comprised of an internal tubular ply made from a synthetic material, a strengthening reinforcement member also made from a synthetic material and an external ply made from a synthetic material. At least one of the plies is bonded, by melting, to the reinforcement member. The strengthening reinforcement member has, over the whole or part of its length, a reticulate lacunary structure, of which only internal and external faces thereof are bonded to corresponding adjacent plies. Spaces of the lacunary structure are blocked off by, but not filled by, these plies.

BACKGROUND OF THE INVENTION

The invention relates to the technical field of pipes used fortransporting gases, fluids, such as water, rainwater, or waste water,but also as a protective sheath, for various mains or conductors. Italso relates more especially to pipes having a diameter lying between 30and 1000 mm.

Currently, most pipes of this type are produced from east iron, steel,cement, or from very thick synthetic material, in order to have thedesired strength. All these pipes have either the disadvantage ofpossessing a high weight per linear meter or, when they are made frompolyvinyl chloride, of polluting the environment, since they containchlorine.

Multilayer pipes are also known which, produced from compositematerials, that is to say from materials of different type, have thedisadvantage of requiring, in order to manufacture them, tools which arecomplicated, expensive and difficult to adjust.

This is the case with multilayer pipes described in GB-A-1,449,753, U.S.Pat. No. 3,977,440 and U.S. Pat. No. 4,044,799 which, being composed ofan internal tubular ply, a strengthening reinforcement member having alacunary structure or otherwise, and an external tubular ply, bond thereinforcement member to at least one of the plies by embedding it, bymelting, in the external ply. The enhancement of the mechanicalcharacteristics of the pipes thus obtained is largely counterbalanced byan increase in the weight of the pipe, increasing the amount ofdeflection when the pipe is laid between two supports. To thesedisadvantages, limiting the application of this manufacturing techniqueto pipes of diameters less than 100 mm, should be added those inherentin the manufacturing means which, requiring several stations andoccasionally several injection heads, are expensive, complicated andtricky to adjust.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome these disadvantagesby providing an impermeable and strong multilayer pipe which has a lowweight per linear meter and can be produced in nonpolluting materialswhich may or may not incorporate recovery products.

For this purpose, in the multilayer pipe according to the invention, thestrengthening reinforcement member has, over the whole or part of itslength, a lacunary structure which is reticulated, only the respectivelyinternal and external faces of which are bonded to the adjacent pliesand the lacunary spaces of which are blocked off, but not filled, bythese plies.

This pipe offers the particular feature of possessing a central lacunaryreinforcement member forming a brace between the adjacent plies and towhich these plies are attached in order to form a perfectly impermeable,lightweight and strong tubular body.

According to the embodiments, the reinforcement member and its plies areproduced in the same synthetic material or in synthetic materials ofdifferent types or colors, coming as virgin granules or mixed withrecycling products.

In one embodiment, the reinforcement member made from synthetic materialhaving a lacunary structure is locally replaced by a uniform layer of asubstance which can be used to form a bellmouth for joining two lengthsof pipe.

The invention also relates to the die for the manufacture of such amultilayer pipe.

For this purpose, this die comprises, going from the inside to theoutside:

on the one hand, a first extrusion assembly forming the inner ply andcomposed of an axial core delimiting, with a sleeve surrounding it, anannular channel joined, upstream, to a material supply channel andrunning out, via its downstream end, toward a chilled sizing-die unit;

on the other hand, a second extrusion assembly forming thelacunary-structure reinforcement member and composed of two sleevessurrounding the first extrusion assembly, and forming a second annularchannel communicating, upstream, with a second material supply channeland, downstream, with an extrusion annulus, at least one of thesesleeves including, at its downstream end, notches interacting with theannulus, this sleeve being connected to means capable of moving it inrelation to the other sleeve in order to alter the cross-section of thisannulus,

and, in addition, a third extrusion assembly forming the outer ply andcomposed of a fixed bush, placed around the second extrusion assemblyand delimiting, with an internal bore made in the die head, a thirdannular channel, the upstream end of which communicates with a thirdmaterial supply channel, whereas its downstream end runs out toward thelacunary reinforcement member pressed against the inner ply.

Such a die makes it possible to produce continuously thelacunary-structure reinforcement member and the two, respectively innerand outer, adjacent plies, sandwiching this reinforcement member, beingpressed against it and being bonded to it during the cooling.

The three material supply channels may be joined to the same extruder orto three different extruders. This independence also enables only two ofthe supply channels to be fed, for example the first and the secondsupply channel or the second and third channel, in order to form a pipewhich includes only one inner ply or only one outer ply associated withthe reinforcement member.

BRIEF DESCRIPTION OF THE DRAWINGS

Characteristics and advantages of the invention will become apparentfrom the description which follows, with reference to the appendeddiagrammatic drawing representing, by way of nonlimiting example, anembodiment of the multilayer pipe and an embodiment of the die formanufacturing it.

FIG. 1 is a partial perspective view with cut-away, showing anembodiment of the multilayer pipe.

FIG. 2 is a transverse sectional side view.

FIG. 3 is a longitudinal sectional view of an embodiment of the dieenabling the multilayer pipe shown in FIG. 1 to be manufactured.

FIGS. 4 to 6 are partial front views showing other embodiments of thereinforcement member.

FIGS. 7 and 8 are partial sectional views showing alternativeembodiments of the die of FIG. 3.

FIG. 9 is a partial front view along the arrow IX of FIG. 7, showing, onan enlarged scale, the jagged slit.

FIG. 10 is a partial side and sectional view showing another embodimentof the die.

FIG. 11 is a partial sectional view along XI--XI of FIG. 10, showing, onan enlarged scale, the extrusion slit of the die assembly B of FIG. 10.

FIG. 12 is a partial plan view, from above, of the pipe obtained usingthis die.

FIGS. 13 to 15 are views similar to those, 10 to 12, showing anotherembodiment of the die and the pipe.

FIG. 16 is a partial sectional view, showing, on an enlarged scale,another embodiment of the multilayer pipe.

FIG. 17 is a partial longitudinal sectional view of a pipe equipped witha fitting endpiece and with a housing for an endpiece.

FIGS. 18 and 19 are front elevation views of the means for forming,respectively, the endpiece and the housing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, the designation 2 denotes a lacunary-structure reinforcementmember made from synthetic material and including, in a known way, ameshed structure delimited by longitudinal filaments or strands 3 andcircular rings 4, these being bonded to the former ones at regularintervals. The designation 5 denotes an inner ply made from syntheticmaterial, whereas the designation 6 denotes an outer ply, also producedfrom synthetic material.

In order to understand the invention, it is specified that theexpressions inner ply and outer ply denote the plies delimiting the wallof the pipe and, depending on the embodiments, these may or may not beadjacent to the central lacunary reinforcement member, or to one of thelacunary reinforcement members of the pipe.

As indicated in the preamble, the material making up the reinforcementmember 2 and the material making up the plies 5 and 6 may be identicalor be different in their type, their color, or in any othercharacteristic. In particular, one of the three aforementioned layersmay be produced in recovery materials. Preferably, and in order to avoidany pollution, the three layers are produced in polyolefins, for examplepolyethylene, which may or may not be recovered.

Owing to its structure, and especially by virtue of the reinforcementmember 2 sandwiched between the two plies 5 and 6, of small thicknessand to which plies the armature is bonded, this pipe has a very highrigidity.

In addition, and by virtue of the placing of the plies 5 and 6 againstand on either side of the lacunary reinforcement member, without fillingthe meshes thereof, the composite pipe thus obtained is very muchlighter than the traditional pipes made from cast iron, cement or madefrom extruded synthetic material.

This pipe is manufactured continuously by means of a die, one embodimentof which is shown in FIG. 3. In this figure, the designations 10 and 11denote die body elements and 12 the die head. These elements are linkedtogether and attached to the stand of an extrusion installation, whichis not shown.

According to the invention, this die comprises a first extrusionassembly A forming the inner ply 5, a second extrusion assembly Bforming the lacunary-structure reinforcement member 2 and a thirdextrusion assembly C forming the outer ply 6.

The first extrusion assembly A is composed of an axial core 13 and acentral sleeve 14 surrounding it. The core 13 is attached to the body 10by its upstream end. It delimits, with the internal bore of the sleeve14, an annular channel 15 which extends over its entire length and isjoined, upstream, to a first material supply channel 16. The annularchannel 15 emerges, downstream, in proximity to the convergent element17a of a chilled sizing-die unit 17, fixed in the extension of the die,in order to cool and to shape the exiting tubular ply 5.

The second extrusion assembly B is constituted by the aforementionedcentral sleeve 14, by an intermediate sleeve 18 surrounding it andfastened to the body 11, and by a sleeve 19 for forming the lacunae,this sleeve being mounted so as to slide over the outer part of theintermediate sleeve 18. The sleeves 14 and 18 delimit between them asecond annular channel 20 communicating, upstream, with a second feedchannel 22 and emerging, downstream, via an annulus 23. In a known way,this annulus is formed between a flange 14a of the sleeve 14 and thedownstream beveled end of the intermediate sleeve 18. It emergesradially toward the outside.

The sleeve 19 includes, at its free end, notches 24. It is connected,via a linkage 25 passing through the bodies 10 and 11, to a drive plate26 placed outside the die and itself connected to motor means capable ofmoving it longitudinally and alternately in the two directions, as shownby the arrow 27.

In this embodiment, the sleeve 19 can occupy two positions, namely:

a downstream partially occulting position, shown in the lower part ofFIG. 3, in which its notches 24 are in the exit plane of the annulus 23in order to form the longitudinal strands 3 of the lacunary structure;

and an upstream totally clear position, shown in the upper part of FIG.3, in which it is moved upstream, enabling the material to exit over theentire periphery of the annulus 23 in order to form, depending on thetime for which it is held in this position, a ring 4 for strengtheningthe strands 3, or a continuous layer of material.

This continuous layer of material, which is locally substituted for thelacunary layer, strengthens the pipe in the zones where it has toundergo a bellmouthing operation, that is to say to be deformed radiallyin order to have a bellmouth which makes it easier to join two pipes byfitting into each other.

The third extrusion assembly C is composed of an outer sleeve 30,fastened to the die head 12 and delimiting, by its outer face and with abore, made in this head, a third annular channel 33. This channel isjoined to an upstream feed chamber 35, itself connected up to a thirdmaterial feed channel 36. The downstream end of the annular channel 33emerges toward the sizing-die unit 17 and thus enables the outer ply 6to be laid down over the reinforcement member 2, itself bearing on theinner ply 5 which flows over the sizing-die unit.

FIG. 3 shows that the inner bore of the outer bush 30 is distinctlyfurther away radially from the blocking-off bush 19 so as not toobstruct the latter's movements.

It is easy to imagine that the three layers exiting the die and bearingon the sizing-die unit 17 have not yet reached their solidificationtemperature and that, because of this, they may be bonded to each otherwithout addition of any other bonding agent.

The feed channels 16-22 and 36 may be joined to the same extruder or toseveral extruders, which makes it possible to produce, using this die,pipes having layers made from synthetic material of the same type or ofdifferent types, and this is so depending on the requirements andconstraints pertaining to the use of this pipe.

Recourse to different extruders also enables recovery plastics to beused in order to constitute, for example, the least visible or the leaststressed layer, preferably the lacunary reinforcement member.

The lacunary structure forming the reinforcement member 2 may have meshcells other than those shown in FIG. 1, for example staggered squaremesh cells, diamond-shaped mesh cells or honeycomb-shaped mesh cells, asshown in FIGS. 4, 5 and 6.

The staggered square mesh cells are obtained by an alternative form ofthe device of FIG. 3, including means, not shown, for pivoting the plate26, the linkage 25 and the sleeve 19 about the longitudinal axis of thedie. This rotation, the angular amplitude of which has a valuesubstantially equal to half the width of one mesh cell, is performed, inone direction or the other, while the sleeve 19 is in the upstreamposition for clearing the annulus 23. It is obvious that, under theseconditions, the linkages 25 pass through the bodies 10 and 11 of the dievia slots, in the form of a circular arc or in another form, leavingthem with the clearance necessary for this rotation.

FIGS. 7 and 8 show alternative embodiments of the die of FIG. 3, thesealternative forms enabling diamond-shaped or honeycomb-shaped mesh cellsto be produced.

In both these alternative forms, the sleeve 19 for forming the lacunaeis associated with a sleeve 40 which, placed downstream of it, is linkedto the means 25 to 27 which move this bush.

In FIG. 7, this link is provided by an axial rod 42, linked to the plate26 and mounted so as to move freely in translation in a bore 41 of thecore 13, and by radial arms 43 placed, with clearance, in radial slots44 and 45 of, respectively, the core 13 and sleeve 14. The arms 43,which cut the annulus 15 and consequently the path of the materialmaking up the layer 5, have, in transverse cross-section, a sharp-edgedlamellar profile in order to reduce to a minimum their effect on theflowing material. Of course, downstream of this perturbation zone, theannulus 15 includes a compression zone enabling the material to absorbthe splitting produced by the arms and to reform a uniform ring.

In the alternative embodiment of FIG. 8, the sleeve 40 is linked to theplate 26 (FIG. 3) by tie rods 43 mounted so as to slide vertically inthe sleeve 14 and linked to an internal collar 44 of this sleeve.

In both these embodiments, and as shown in FIG. 9, the facing edges of,respectively, the sleeve 19 and the sleeve 40 are equipped withimbricating dentations, respectively 45 and 46, and formed by triangularteeth separated by "V"-shaped notches and delimiting between them ajagged slit 47.

During the production of the lacunary structure of the reinforcementmember 2, the sleeve 19 and the sleeve 40 are simultaneously set into amotion of longitudinal movement, alternately downstream and thenupstream, so as to move the jagged slit 47 made between the twodentations between a downstream position, shown in FIG. 9, in which whatmay be called the crest 47a of the teeth of the jagged slit 47 is in theregion of the annulus 23 and an upstream position in which it is theroots 47b which are in the region of this annulus. Between these twopositions, the material exiting via the annulus 23 is distributed overthe branches 47c between crests and roots and thus forms thediamond-shaped mesh shown in FIG. 5.

The honeycomb-shaped mesh cells, shown in FIG. 6, are obtained by one orother of the dies of FIGS. 7 and 8, temporarily stopping thelongitudinal movement of the sleeves 19 and 40 at each of the ends oftheir travel in order to form, as shown in FIG. 6, bars 48 connectingthe strands 49 formed by the branches 47c of the jagged slit 47.

FIGS. 10 and 11, and 13 and 14, show two other embodiments of the secondextrusion assembly B, enabling lacunary structures with diamond-shapedmesh cells to be obtained. In FIGS. 10 and 11, the assembly B iscomposed of two coaxial sleeves 50, 51 delimiting between them thesecond annular channel 20. The extrusion annulus 52 is delimited betweentwo divergent frustoconical bearing surfaces made on extreme anddownstream flanges 53 and 54 of, respectively, the sleeve 50 and thesleeve 51. Radial notches 55 and 56 (FIG. 11), angularly spaced with thesame angular spacing, emerge into each of the conical bearing surfaces.

Each of the two sleeves 50 and 51 is connected to means, which are notshown but are known to the person skilled in the art, capable ofpivoting it in the reverse direction of the other sleeve and, forexample, and as shown in FIG. 11, in the direction of the arrow 57 forthe sleeve 50 and in that of the arrow 58 for the sleeve 51. Duringthese rotations, each notch 55 and 56 is partially occulted insuccession by the facing conical bearing surface, as shown by thecontinuous lines in FIG. 11, or completed by the facing notch, as shownby the broken lines in the same figure. When the notches are partiallyocculted, they form the inclined strands of the diamond-shaped meshcells of the lacunary structure shown lacuna! FIG. 12 and when they arein coincidence with the facing notches they form the points ofintersection of the mesh cells.

As in the embodiments of FIGS. 7 and 8, a temporary interruption in therotation of the two sleeves 50 and 51 enables longitudinal strands to beobtained, giving a honeycomb-shaped mesh.

In FIGS. 13 and 14, the extrusion assembly B is composed of an outersleeve 60 and an inner sleeve 61 coaxial with the previous one anddelimiting, with it, the second annular channel 20. The extrusionannulus 62 is formed between the downstream end of the internal bore 63of the sleeve 60 and the periphery of an annular knife 64. The knife hasan external diameter smaller than the internal diameter of the bore 63and is mounted at the end of a tubular support 65 with offsetting of itslongitudinal axis in relation to that of this support. This support isplaced inside the sleeve 61, between this sleeve and that one 14 of thefirst extrusion assembly A. The support is connected, via its upstreampart, to means, not shown, capable of driving it rotationally about thesleeve 14. The outer sleeve 60 includes, at the downstream end of itsinternal bore, longitudinal blind notches 66 emerging into this bore andfrom the end of the sleeve (FIG. 14). These notches are spaced apart bya constant angular spacing.

In operation, in the part of the annulus 62 where the annular knife 64is in contact with the bore 63, as shown in the upper part of FIG. 14,only the notches 66 output longitudinal strands of material, whereas, inthe opposite part of the annulus 62, the material is output by thenotches 66, but also by the annulus 62 which, by the motion of the knife64, forms a helical strand inside the imaginary cylinder delimited bythe longitudinal strands. The lacunary structure thus obtained is shownin FIG. 15.

It will be noted that similar results may be obtained by producing thenotches 66 on the periphery of the knife 64 and by providing theexternal sleeve 60 with a smooth bore over its entire length.

In an alternative form of this embodiment, the annular knife 64 iscoaxial with its tubular support 65 which is then mounted so as torotate freely on a bearing surface of the sleeve 14, this bearingsurface itself being offset in relation to the longitudinal axis of thissleeve.

In another alternative form, with a knife 64 coaxial with the support65, the support is mounted with radial clearance between the sleeves 14and 61 and is connected to means capable of imparting to it a nutation,that is to say a substantially pendular motion moving the periphery ofthe knife against the bore 63 of the external sleeve 60.

FIG. 16 shows an embodiment of a five-layer multilayer pipe. This pipeincludes, between the strengthening reinforcement member 2, bonded tothe outer ply 6 and the inner ply 5, an intermediate ply 5a and a secondstrengthening reinforcement member 2a. The intermediate ply 5a isbonded, by melting, to the second strengthening reinforcement member 2awhich is, itself, bonded, by melting, to the inner ply 5.

The replacement of a single lacunary reinforcement member by twolacunary reinforcement members enables the transverse dimensions of thestrands of the lacunary structures to be reduced. This has the advantageof transferring the quantity of heat accumulated in the strands, and inparticular in those coming into contact with the respectively inner 5and outer 6 plies and thus of facilitating the cooling of these plies.In practice, this multilayer structure prevents the formation ofshrinkage zones visible on the surface of the plies and enables smoothand continuous surfaces to be obtained.

Preferably, the meshes of the two lacunary-structure strengtheningreinforcement members 5 and 5a are offset, transversely andlongitudinally, so that the points of intersection of one reinforcementmember are between the mesh cells of the other reinforcement member andthis is the case so as, on the one hand, to distribute the heatconcentration zones on exiting from the die and, on the other hand, toimprove the strength of the composite obtained.

FIG. 17 shows a three-layer pipe obtained by cutting off into a lengththe product exiting from any one of the dies described hereinabove andshaped in order to allow its connection, by fitting, to an identicalpipe. This pipe includes, at one of its ends, a fitting endpiece 70 and,at its other end, a housing 71 for receiving this endpiece.

The endpiece 70 has an outer diameter less than the outer one of thepipe, whereas the housing 71 has an inner diameter greater than theinner one of this same pipe and equal, to within the functionaltolerance, to the outer one of the endpiece.

This endpiece 70 is obtained by raising the corresponding end of thepipe to a temperature at least equal to the softening point of thematerials of which its layers are composed and by engaging it in thearrangement shown in FIG. 18, and composed of an axial ram 72 and jaws73. The ram 72 has an inner diameter equal to that of the pipe. The jaws73 can move radially, in the direction of the arrow 74, between aposition in which they accept the engagement of the end of the pipe andan end-of-compression position. While they are being clamped onto theend of the pipe, they squash the component strands of the strengtheningreinforcement member 2, thereby laminating them between the plies 5 and6, which also undergo an at least partial lamination.

This compression-lamination is sufficiently strong to form, if this isnecessary, and by means of molding cavities 75 made in the jaws, one ormore tenons 76 projecting radially from the endpiece.

At the end of the operation, the endpiece is composed of a generallysolid reinforced-wall.

The preparation of the housing 71 is carried out under the sameconditions, but by means of the arrangement shown in FIG. 19, andcomprising an external collar 77 and an internal ram 78. The end of thepipe is engaged in the collar 77, which ensures its external sizing,like the ram 72 ensures the internal sizing of the endpiece 70. The endis deformed radially by insertion into its internal bore of the ram 78which has an external diameter greater than the internal one of the pipeand equal to the internal one of the housing 71. This radial force alsocauses the compression and the lamination of the strands of thestrengthening reinforcement member 2 between the plies 5 and 6.

When the housing 71 is equipped with longitudinal grooves 79, withlocking catch 80 for the tenons 76, these grooves and catches are formedby mold cavities 81 which can move radially in the ram 78 and areconnected to means capable of moving them between a retracted positionin the mandrel and a working position, in which they project from thismandrel and locally deform the reinforced wall produced at this end.

It will be noted, by virtue of its lacunae, that the strengtheningreinforcement member 2 makes it possible to shape the ends of the pipeby using pressures which can be obtained by simple, reliable andinexpensive mechanical means, and to obtain, in the fitting zones notrequiring deformability, rigid and strong walls promoting the connectionof the pipes.

The possibility of producing, on the endpieces 70 and housings 71 whilethey are being formed, means 76, 79, 80 which subsequently ensure thepositive connection of the various lengths, is particularly useful forpipes comprising an inner ply and/or an outer ply made frompolyethylene, known to be difficult to join by adhesive bonding.

In an alternative form, the grooves 79 are in the form of helices. Thisarrangement makes it possible, by imparting a longitudinal thrust, forexample by means of the shovel of a shovel loader, to the free end of apipe, to impart to its other end the translational and rotational motionthat favors the locking of tenons 76 in the notches 80. This arrangementis particularly advantageous for pipes that have a diameter greater than400 mm and are difficult for just one man to be able to grasp.

The pipe according to the invention may be used for transporting gases,fluids, or sounds, but also for constituting a protective sheath forvarious mains or conductors. Whatever its application, for a strengthequal to that of current pipes made from various materials, it islighter in weight which makes it possible to envisage using it startingfrom very small diameters of the order of 30 mm up to diameters of theorder of 1000 mm which, hitherto, could not be envisaged in lengths ofpipe of standard length, on account of their weight.

We claim:
 1. A multilayer pipe comprising at least one internal ply made from a first synthetic material, a strengthening reinforcement member made from a second synthetic material having a lacunary structure and one external ply made from a third synthetic material, in which at least one of the internal and external ply is bonded, by melting, to the reinforcement member, and wherein the strengthening reinforcement member has over at least part of its length, a lacunary structure which is reticulated, wherein at least one of internal and external faces of the strengthening reinforcement member is bonded continuously to adjacent plies and lacunary spaces of which are blocked off, but not filled, by the adjacent plies.
 2. The pipe as claimed in claim 1, which possesses only one strengthening reinforcement member included between the internal and external plies and bonded thereto.
 3. The pipe as claimed in claim 1, wherein the reinforcement member and one of the internal ply and the external ply to which the reinforcement member is bonded are produced from an identical synthetic material.
 4. The pipe as claimed in claim 1, wherein at least one of the internal and external plies is produced from a synthetic material other than that of the reinforcement member.
 5. The pipe as claimed in claim 1, wherein the lacunary-structure of the reinforcement member transitions to a generally solid layer of material at least one end of the pipe.
 6. The pipe as claimed in claim 1, further comprising:at one end thereof, a fitting endpiece of an outer diameter less than an outer diameter of the pipe, and having a reinforced wall formed by engagement of the one end, the one end being raised to the softening point of the materials constituting layers of the multilayer pipe, between an axial ram and radial outer jaws capable of compressing the pipe, thereby squashing and laminating the strengthening reinforcement member and the external ply against the internal ply; and at an opposite other end thereof, a housing for receiving the endpiece having an internal diameter greater than an internal diameter of the pipe and including a reinforced wall formed by engagement of the other end, the other end being raised to the softening point of the materials constituting the layers of the multilayer pipe and placed in an external collar of an axial ram capable of compressing the other end radially, thereby squashing and laminating the strengthening reinforcement member and the internal ply against the external ply.
 7. The pipe as claimed in claim 6, wherein the endpiece includes at least one tenon projecting radially outward whereas the housing includes at least one groove that communicates with a transverse catch for locking the tenon.
 8. The pipe as claimed in claim 1, further comprising, between the strengthening reinforcement member and the internal ply, an intermediate ply bonded, by melting, on one side to the reinforcement member and on an other side to a second lacunary reinforcement member, the second lacunary reinforcement member being itself bonded, by melting, to the internal ply.
 9. The pipe as claimed in claim 8, wherein mesh cells of the second lacunary reinforcement member are offset laterally and transversely in relation to mesh cells of the first reinforcement member. 